A block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene and having a relatively high content of the vinyl aromatic hydrocarbon is widely used for an application to injection molding or for an application to extrusion, such as sheets and films, by virtue of its excellent characteristics such as transparency and impact resistance. Particularly, some such block copolymers and styrene polymer compositions having such a block copolymer blended therewith, which are excellent in transparency, impact resistance, etc., have been proposed.
For example, JP-A-52-78260 discloses a linear copolymer composition wherein the molecular weight distribution at a vinyl aromatic hydrocarbon block moiety is from 2.3 to 4.5 and a branched block copolymer composition wherein the molecular weight distribution at a vinyl aromatic hydrocarbon block moiety is from 2.8 to 3.5, which is produced by blending. JP-B-2-59164 discloses a linear block copolymer and a branched block copolymer, wherein the molecular weight distribution at a vinyl-substituted aromatic hydrocarbon block moiety is from 1.2 to 2.0. JP-A-53-286 discloses a branched block copolymer wherein the ratio of the number average molecular weight of a high molecular weight component to a low molecular weight component at a vinyl-substituted aromatic hydrocarbon block moiety in two linear copolymers prior to coupling is from 3 to 7, and its production process. JP-A-7-173232 discloses a branched block copolymer having at least three polymer blocks each comprising a vinyl aromatic hydrocarbon as monomer units, and its production process. Further, JP-A-57-28150 discloses a method of combining a branched block copolymer wherein the molecular weight distribution at a vinyl aromatic hydrocarbon block moiety is out of the range of from 2.8 to 3.5.
However, such block copolymers and compositions of such a block copolymer with a thermoplastic resin formed by the above methods are poor in balance of transparency, impact resistance, etc. Particularly in injection molding, molding is carried out under a high shearing force, whereby a molded product is likely to have anisotropy and tends to be poor in strength in a certain direction, and a sufficient molded product can hardly be provided.